A solid high polymer fuel cell is formed by laminating positive and negative electrode catalyst layers (cathode and anode) on both sides of an electrolyte membrane made of ion exchange resin or the like, and further laminating gas diffusion layers on these electrode catalyst layers to form an electrode structure, which is called a unit cell. Plural unit cells are laminated on both sides of a separator, and a practical fuel cell stack is formed. The separator is made of a material having an electron transmitting function, and has multiple gas passages formed like grooves for independently circulating fuel gas using hydrogen and oxidizer gas such as oxygen or air, and is placed between unit cells in a state contacting with the gas diffusion layer.
In such fuel cells, for example, by circulating hydrogen gas as a fuel gas in the gas passage of the separator at the negative electrode side, and circulating oxidizing gas such as oxygen or air in the gas passage of the separator at the positive electrode side, an electrochemical reaction takes place, and electricity is generated. During generation of electricity, the gas diffusion layer transmits electrons generated by electrochemical reaction between the electrode catalyst layer and the separator, and diffuses fuel gas and oxidizing gas at the same time. The electrode catalyst layer at the negative electrode side induces a chemical reaction in the fuel gas, and generates protons and electrons, while the electrode catalyst layer at the positive electrode side produces water from oxygen, protons, and electrons, and the electrolyte membrane transmits protons ionically. Thus, electrical power is drawn out through the positive and negative electrode catalyst layers.
Hitherto, the separator was mainly made of graphite material, and the gas passages were formed by cutting grooves. Graphite materials include gas impermeable graphite having resin such as phenol resin impregnated in baked isotropic graphite, amorphous carbon having resin such as phenol resin baked after forming, and composite material made of resin and graphite. These graphite materials are high in hardness, and it was difficult to form gas passages, or mechanical strength and impact resistance were poor.
In light of such problems, recently, it has been proposed to use new materials that can overcome the problems of the graphite materials, such as press-formed materials of thin metal plates of aluminum, titanium, stainless steel, or the like. Among these, stainless steel has a passive film on the surface and is superior in corrosion resistance. However, when the stainless steel is used in the separator of a fuel cell, catalyst poisoning or conductivity reducing of electrode membrane may be caused by eluting ions. Moreover, since the electrical resistance of the passive film is high, the contact resistance increases at the contact interface of the separator and the electrode structure.
As means for solving these problems, a separator made of gold-plated stainless steel was proposed in Japanese Patent Application Laid-open No. 10-228914. It has also been attempted to enhance the corrosion resistance and conductivity by precipitating conductive boride or boron carbide from inside stainless steel, and exposing the precipitates on the surface together with the passive film.
Of these conventional means of solution, the former method incurs a very high manufacturing cost. Alternatively, if the gold plating is exposed to friction by vibration or the like, the gold plating is likely to peel off at the interface with the stainless steel, and it is not suited to long-term use. Moreover, if there is a pin hole or other defect, corrosion originates therefrom. In the latter means, on the other hand, the material becomes brittle due to precipitates appearing on the surface, and when bent by press forming, the precipitates separate or fall off from the bent portion, and corrosion is initiated from the fall off marks, and this is also not suited to long-term use.